Project Summary Coronary microvascular disease (CMD) is increasingly recognized as a major contributor to heart failure in patients with obesity and diabetes. Recent studies have shown that, in the absence of obstructive epicardial coronary artery disease (CAD), impaired myocardial perfusion reserve (MPR) assessed by PET or MRI is a functional consequence of CMD and predicts a high risk of adverse cardiac events. Presently, unlike for CAD, there is no established treatment for CMD. Mechanistically, obesity and diabetes cause activation of the renin- angiotensin-aldosterone system and, in addition to angiotensin II, basic research demonstrates a central role of aldosterone and the corresponding mineralocorticoid receptor in vascular oxidative stress and inflammation, resulting in endothelial dysfunction, impaired vascular smooth muscle function and vasodilation, and perivascular fibrosis in excised isolated arterioles. Translating these findings, initial clinical imaging studies demonstrate that mineralocorticoid receptor antagonists (MRAs) improve impaired MPR in overweight/obese and diabetic patients without CAD. While promising, the specific mechanism of action underlying the therapeutic effects of MRAs in improving MPR is not fully understood, data suggest that MRAs may be ineffective in women, and there are conflicting data regarding whether the therapeutic effects extend to microvascular dysfunction in peripheral artery disease. We have developed reproducible quantitative perfusion MRI methods for mice and established mice fed a high-fat diet (HFD) as a model of obesity, type II diabetes, CMD without CAD, and impaired MPR. We also showed that, like in humans, eplerenone (EPL), a selective MRA, improves MPR in male HFD mice. The goals of the present proposal are to develop and use MRI and pharmacologic and genetic manipulations in HFD mice to prove the central roles of the vascular smooth muscle mineralocorticoid receptor and oxidative stress in impaired MPR due to obesity and diabetes, and to shed light on open questions concerning whether MRAs are effective in females and in peripheral microvascular dysfunction.